The present invention relates to a multi-channel access (MCA) radio telephone system and, more particularly, to a method for clearing such an MCA radio telephone system.
In a conventional MCA radio telephone system, such as a marine telephone system, a plurality of shore subscriber telephone sets are connected through a shore telephone exchange and a concentrator to a base station, which is coupled to a plurality of substations, such as ships, over a plurality of radio channels. In such a system, when a shore subscriber hangs up first, the clearing process can be accomplished by either one of the following two methods.
In the first method, in response to the hanging-up of the shore subscriber, the shore exchange sends a busy tone through the concentrator to the base station. The busy tone is further carried with the present radio channel from the base station to the substation the subscriber of which can hear the busy tone as an audible signal indicating that the shore subscriber had hung up. The substation subscriber, therefore, hangs up his own telephone set. Only then is the radio channel released, and thereby made available for use by another substation.
A disadvantage of this method is that the radio channel remains busy until the substation subscriber hangs up even though the call is finished. This is detrimental to the efficient utilization of radio channels in an MCA system, wherein these radio channels are shared by a plurality of substations.
In the second method, in response to the hanging-up of the shore subscriber, the shore exchange sends a busy tone to the concentrator which detects this busy tone and finds that the shore telephone is already on-hook. Also, the concentrator sends to the substation a signal (disconnection signal) to let it know that the shore subscriber has hung up. This disconnect signal releases the radio channel shared by a combination of the base station and the substation, and further releases the direct current (D.C.) loop of the line connecting the subscriber to the shore exchange. Upon detection of the disconnect signal, the substation releases its radio channel and causes an internal circuit therein to generate a busy tone (a local busy tone), which is heard in the telephone set. This local busy tone keeps on sounding until the substation subscriber hangs up.
This method is favorable because it is a more efficient utilization of radio channels. Nevertheless, it has a disadvantage since the D.C. loop of the subscriber line is open as viewed from the shore exchange. Although the substation is deemed to have already hung up and to be ready to accept a call from the exchange, at any time, the telephone set connected to the substation is actually still off-hook, so that no call can be received as yet.